title: Estimates of the number of human infections with influenza A(H3N2) variant virus, United States, August 2011-April 2012.
abstract: BACKGROUND
  Thirteen human infections with an influenza A(H3N2) variant (H3N2v) virus containing a combination of gene segments not previously associated with human illness were identified in the United States from August 2011 to April 2012. Because laboratory confirmation of influenza virus infection is only performed for a minority of ill persons and routine clinical tests may not identify H3N2v virus, the count of laboratory-confirmed H3N2v virus infections underestimates the true burden of illness.
  
  
  METHODS
  To account for this underascertainment, we adapted a multiplier model created at the beginning of the influenza A(H1N1) 2009 pandemic to estimate the true burden of H3N2v illness. Data to inform each of these parameters came from the literature and from special projects conducted during the 2009 H1N1 pandemic and the 2010-2011 influenza season. The multipliers were calculated as the simple inverses of the proportions at each step, and we accounted for variability and uncertainty in model parameters by using a probabilistic or Monte Carlo approach.
  
  
  RESULTS
  We estimate that the median multiplier for children was 200 (90% range, 115-369) and for adults was 255 (90% range, 152-479) and that 2055 (90% range, 1187-3800) illnesses from H3N2v virus infections may have occurred from August 2011 to April 2012, suggesting that the new virus was more widespread than previously thought.
  
  
  CONCLUSIONS
  Illness from this variant influenza virus was more frequent than previously thought. Continued surveillance is needed to ensure timely detection and response to H3N2v virus infections.
authors.affiliation: Epidemiology and Prevention Branch, Influenza Division, National Center for Immunization and Respiratory Diseases, Centers for Disease Control and Prevention, 1600 Clifton Rd. NE, Atlanta, GA 30333, USA. mbiggerstaff@cdc.gov
authors.forename: Matthew
authors.initials: M
authors.lastname: Biggerstaff
authors.forename: Carrie
authors.initials: C
authors.lastname: Reed
authors.forename: Scott
authors.initials: S
authors.lastname: Epperson
authors.forename: Michael A
authors.initials: MA
authors.lastname: Jhung
authors.forename: Manoj
authors.initials: M
authors.lastname: Gambhir
authors.forename: Joseph S
authors.initials: JS
authors.lastname: Bresee
authors.forename: Daniel B
authors.initials: DB
authors.lastname: Jernigan
authors.forename: David L
authors.initials: DL
authors.lastname: Swerdlow
authors.forename: Lyn
authors.initials: L
authors.lastname: Finelli
mesh_terms: D000328:Adult; D000818:Animals; D002648:Child; D004196:Disease Outbreaks; D005260:Female; D006801:Humans; D053118:Influenza A Virus, H1N1 Subtype / Q000235:genetics; D053122:Influenza A Virus, H3N2 Subtype / Q000235:genetics / Q000302:isolation & purification*; D007251:Influenza, Human / Q000453:epidemiology* / Q000635:transmission; D008297:Male; D009976:Orthomyxoviridae Infections / Q000453:epidemiology / Q000635:transmission / Q000662:veterinary; D011159:Population Surveillance; D013552:Swine; D013553:Swine Diseases / Q000453:epidemiology / Q000635:transmission; D014481:United States / Q000453:epidemiology
keywords: influenza; influenza A(H3N2) variant; prevalence